Powder coatings are promoted as environmentally benign coating systems in that no solvent is used during the application of the coating to the substrate. Powder coating formulations are quite expensive, however, primarily as a result of the extensive processing required to generate the powder. During such processing, resin, which is typically either an acrylate or polyester, is blended with a variety of additives (for example, pigments, stabilizers, fillers) and then extruded to generate an intimate mixture. The efflux from the extruder is pelletized, then ground, then sieved to recover only the smaller particles. The extensive processing of powder coatings and the relatively low yield (as a result of the sieving step) results in a per-pound cost typically in excess of $5. The expensive nature of powder coating has been a substantial factor in preventing powder coatings from capturing more of the overall coatings market.
Attempts to improve the processing of powder coatings have met with limited success. For example, U.S. Pat. Nos. 5,399,597 and 5,548,004 disclose a process purported to generate powder coating formulations via processing with supercritical carbon dioxide (CO.sub.2). In general, the process comprises the steps of (a) adding all of the mix components to a pressure vessel, (b) adding CO.sub.2 at a supercritical pressure and temperature, (c) stirring the components within the pressure vessel; and (c) subsequently blowing the contents through a nozzle into a second vessel maintained at a lower pressure, thereby exploding the mixture into droplets.
There are a number of potential problems with the process of U.S. Pat. Nos. 5,399,597 and 5,548,004. For example, explosion of the contents of the pressure vessel in that process may result in an undesirably broad distribution of particle sized in the resulting powder coating. Moreover, control of the process via control of the aperture size of the nozzle is likely to be difficult. Furthermore, very high pressures are required.
European Patent Application No. 95308726.9 discloses a process in which supercritical CO.sub.2 and typically a cosolvent are purportedly used to dissolve some components of a coating powder. The mixture is then flashed through a valve, causing rapid precipitation of a powder. A substantial problem exists with the process of European Patent Application No. 95308726.9 in that some of the starting components are partially soluble in CO.sub.2 (in the case of resin, for example, only a part of the molecular weight distribution), while others are completely insoluble in CO.sub.2 (for example, the pigments and other inorganic components). It is thus likely that fractionization of the resin will occur under the described process conditions when using only CO.sub.2 as a solvent. Moreover, the use of cosolvents such as tetrahydrofuran (THF) to reduce fractionization through creation of a single phase can render the process environmentally unfriendly as a result of residual solvent in the powder coating. Residual solvent can also make particles stick together. Furthermore, dissolving the starting materials in a single phase is also likely results in submicron sized particles which are undesirably small for use in powder coatings.
Given the limitations of current processes for the generation of powder formulations, it is very desirable to develop improved processes for the generation of powder formulations.